You almost never die, you are #1 in heal and you don't go oom, not even in 24/7 zerg games (I could easily get 2000 spirit more on my monk, but don't need it. Additionally you can stack 20 mana tea before the start of each game for an additional 80% mana - blizzard should fix this). So MW is really strong in RBG (already was strong before 5.2).
You suck in arena though, so if you just want to do RBG MW is perfectly fine; if you want to play arena you should consider if this is the right choice.